EP0456973A2 - Synchronisation apparatus - Google Patents
Synchronisation apparatus Download PDFInfo
- Publication number
- EP0456973A2 EP0456973A2 EP91102981A EP91102981A EP0456973A2 EP 0456973 A2 EP0456973 A2 EP 0456973A2 EP 91102981 A EP91102981 A EP 91102981A EP 91102981 A EP91102981 A EP 91102981A EP 0456973 A2 EP0456973 A2 EP 0456973A2
- Authority
- EP
- European Patent Office
- Prior art keywords
- sequence
- circuit
- bits
- predicted
- predetermined number
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Withdrawn
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04J—MULTIPLEX COMMUNICATION
- H04J3/00—Time-division multiplex systems
- H04J3/02—Details
- H04J3/06—Synchronising arrangements
- H04J3/0602—Systems characterised by the synchronising information used
- H04J3/0605—Special codes used as synchronising signal
- H04J3/0608—Detectors therefor, e.g. correlators, state machines
Definitions
- This invention relates to apparatus for detecting a synchronisation sequence of data bits in a bit stream, where the sequence forms part of a cyclical linear feed back sequence.
- the apparatus may, for example, be used for frame synchronisation.
- Synchronization in a bit error analyser is a continous process used for the purposes of error measurements.
- means are provided for detecting a predetermined third sequence of bits within the first sequence, after the second sequence has been locked.
- the third sequence may be used for frame synchronization.
- the feed forward register comparator will always be correct, unless the received data contain bit errors.
- the feed back comparator 12 will show correct bits if the generated sequence is synchroneous with the received sequence, and the received sequence is free from errors, otherwise only 50% in average will be correct.
- N A certain number (N) of consecutive correct bits are counted from the feed forward comparator 12 and when a terminal count is generated, the content of the feed forward register is downloaded to the feed back register. If it was in fact the correct sequence which was found the feed back is now synchronized with the transmitter sequence, and the last 13 bits in the sequence can be detected free from errors by comparator 19, and the frame synchronization timing found.
- the number of bits (N) counted in the feed forward comparator is chosen fairly low, in order to get a large number of possible synchronizations, but not large enough to avoid falsing. Therefore correct bits from the feed back comparator are also counted, and synchronization is not accepted unless a larger number of correct bits are counted.
- the performance of the invention depends very much on these two figures. A figure of 9 to 10 correct bits from the feed forward register will give a maximum number of correct synchronizations. Also the number of false synchronizations will increase, but with a sufficient long sequence the feed back register will have enough time to cancel these.
- errors may be allowed, for example a predetermined number of errors per N or M counts.
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- Engineering & Computer Science (AREA)
- Computer Networks & Wireless Communication (AREA)
- Signal Processing (AREA)
- Synchronisation In Digital Transmission Systems (AREA)
- Detection And Prevention Of Errors In Transmission (AREA)
Abstract
This invention relates to apparatus for detecting a synchronisation sequence of data bits in a bit stream, where the sequence forms part of a cyclical linear feed back sequence. The apparatus may, for example, be used for frame synchronisation. It comprises: a first linear feedback circuit (11) for predicting the next received data bit and checking whether it is predicted correctly, a second linear feedback circuit (14) for generating the same second sequence, and thereby also predicting the next received data bit; means (13, 16) for setting the second circuit to the same position in the sequence as the first circuit when a first predetermined number of bits (N) has been received as predicted by the first circuit, and periodically thereafter when a new bit is received as predicted by the first circuit; means (18) for determining when a second predetermined number (M) of bits has been received as predicted by the second circuit, and means (16) for ceasing setting of the second circuit at that time, whereby the second circuit is locked to continue generating the sequence of bits to which it is set.
Description
- This invention relates to apparatus for detecting a synchronisation sequence of data bits in a bit stream, where the sequence forms part of a cyclical linear feed back sequence. The apparatus may, for example, be used for frame synchronisation.
- In the field of bit error analysers, the Hewlett Packard 1645A bit error analyser provides a feed forward shift register and a feed back shift register for analysing a linear feed back sequence for the purposes of bit error rate measurements. The output of the feed forward register is compared with received data in an Exclusive OR gate. If an error is present in the received data, the data will be different from the feed forward data, providing a feed forward error signal. The feed back register is parallel loaded from the feed forward register when a clock slip is detected. This results in automatic synchronization of the feed back register with the incoming data. The feed back register taps are combined through an Exclusive OR gate and fed back to the beginning of the shift register. The feed back register output is compared with the received data and the resulting comparison provides a feed back error signal.
- Synchronization in a bit error analyser is a continous process used for the purposes of error measurements.
- It is an object of the present invention to provide synchronization to a bit stream in spite of the existence of errors in the synchronization sequence.
- According to the present invention, there is provided apparatus for detecting a first synchronization sequence of data bits in a bit stream, where the sequence forms part of a second cyclical linear feed back sequence, the apparatus comprising: a first linear feed back circuit for predicting the next received data bit and checking whether it is predicted correctly, a second linear feed back circuit for generating the same second sequence and thereby also predicting the next received data bit; means for setting the second circuit to the same position in the sequence as the first circuit when a predetermined number of bits (N) has been received as predicted by the first circuit, and periodically thereafter when a new bit is received as predicted by the first circuit; means for determining when a second predetermined number (M) of bits has been received as predicted by the second circuit, and means for ceasing setting of the second circuit at that time, whereby the second circuit is locked to continue generating the sequence of bits to which it is set.
- It is particularly preferred that means are provided for detecting a predetermined third sequence of bits within the first sequence, after the second sequence has been locked. The third sequence may be used for frame synchronization.
- The synchronization of two bit streams and the use of the last bits in the sequence as a frame synchronization signal is in particular a feature not found in the prior art.
- Fig 1 shows a circuit diagram of apparatus in accordance with the preferred embodiment of the invention.
- The purpose of the invention is to detect the beginning of a data telegram (frame synchronization) This is usually done by detecting a certain sequence of bits. The sequence must have a certain length, e.g. 32 bits, in order to avoid falsing, but when the sequence is made longer the probability of detecting the sequence will be smaller, unless the signal is completely free from errors. In this invention the sequence can be made as long as one may wish, and the probability of detecting frame synchronization will increase with the length instead of decrease. Falsing will be independent of the length.
- The circuit shown in Figure 1 shows a feed
forward register 10 with an associatedfeed back circuit 11 for predicting the next bit of the linear feed back sequence. An Exclusive ORgate 12 is provided for comparing the data input with the results of thefeed back circuit 11. Afirst counter 13 counts the 'zeros' from thegate 12. Afeed back register 14 is provided with afeed back circuit 15 similar to feedback circuit 11. Agate 16 is connected between the feedforward register 10 and thefeed back register 14 for parallel loading the contents of the feedforward register 10 into thefeed back register 14. A second Exclusive ORgate 17 compares the results of thefeed back circuit 15, which predicts the next bit of the linear feed back sequence, with the data input. Asecond count circuit 18 counts the 'zeros' from the Exclusive ORgate 17. Theparallel load gate 16 is under the control of thecounters bit comparator 19 receives and compares the output from thefeed back register 14 and searches for a predetermined sequence of bits. When the predetermined sequence is detected, a frame synchronization detect signal is provided by thecomparator 19. Agate 20 is provided under the control of thecounter 18 for blocking the detector signal from thecomparator 19. - The function of the circuit is as follows.
- The sequence is generated in the transmitter by a LFSR (Linear Feed Back Shift Register) with
length 13 bits. The repetition length is 8191 bits (2¹³⁻¹) but other feed back circuits with a longer sequence can be used, and the sequence can be shortened by loading an appropriate word into the register before it is started. In this example 13 bits are sufficient to define the position in the sequence, because all combinations of 13 bits are generated, except all zeros. - In the receiver are two LFSRs, one with the feed back circuit unconnected (this is the feed forward register 10) and one with the feed back connected to the input (this is the feed back register 14). The output from each register is compared with the incoming datastream, and the number of correct bits counted.
- If the input data are random, 50% in average will be correct, but if the data contain the preamble sequence, the feed forward register comparator will always be correct, unless the received data contain bit errors. The
feed back comparator 12 will show correct bits if the generated sequence is synchroneous with the received sequence, and the received sequence is free from errors, otherwise only 50% in average will be correct. - A certain number (N) of consecutive correct bits are counted from the feed
forward comparator 12 and when a terminal count is generated, the content of the feed forward register is downloaded to the feed back register. If it was in fact the correct sequence which was found the feed back is now synchronized with the transmitter sequence, and the last 13 bits in the sequence can be detected free from errors bycomparator 19, and the frame synchronization timing found. - The number of bits (N) counted in the feed forward comparator is chosen fairly low, in order to get a large number of possible synchronizations, but not large enough to avoid falsing. Therefore correct bits from the feed back comparator are also counted, and synchronization is not accepted unless a larger number of correct bits are counted. The performance of the invention depends very much on these two figures. A figure of 9 to 10 correct bits from the feed forward register will give a maximum number of correct synchronizations. Also the number of false synchronizations will increase, but with a sufficient long sequence the feed back register will have enough time to cancel these. A figure of 22 correct bits (M) from the feed back comparator has been found to give a suitable compromise between falsing and probability of receiving frame synchronization in a noisy signal. If falsing is critical a higher figure can be chosen without affecting the synchronization too much. N and M could, for example lie in the ranges 8 to 16 and 18 to 32 respectively.
- In counting N and M, errors may be allowed, for example a predetermined number of errors per N or M counts.
- Allowing errors may improve the probability of synchronization, but in order to keep the falsing on the same level, N and M would have to be increased, N could for example lay in the
range 12 to 16 with 1 or 2 errors, and M in the range 24 to 32 with 1 to 4 errors.
Claims (7)
- Apparatus for detecting a first synchronisation sequence of data bits in a bit stream, where the sequence forms part of a second cyclical linear feedback sequence, the apparatus comprising:
a first linear feedback circuit for predicting the next received data bit and checking whether it is predicted correctly,
a second linear feedback circuit for generating the same second sequence, and thereby also predicting the next received data bit;
means for setting the second circuit to the same position in the sequence as the first circuit when a first predetermined number of bits (N) has been received as predicted by the first circuit, and periodically thereafter when a new bit is received as predicted by the first circuit;
means for determining when a second predetermined number (M) of bits has been received as predicted by the second circuit, and means for ceasing setting of the second circuit at that time, whereby the second circuit is locked to continue generating the sequence of bits to which it is set. - Apparatus according to claim 1 further comprising means for detecting a predetermined third sequence of bits within the first sequence, after the second circuit has been locked.
- Apparatus according to claim 2 further comprising means for dividing the bit stream into frames dependent on the means for detecting the third sequence of bits.
- Apparatus according to any one of the preceding claims wherein the first predetermined number is in the range 8 to 16.
- Apparatus according to any one of the preceding claims, wherein the second predetermined number is in the range 18 to 32.
- Apparatus according to any one of the preceding claims comprising means for counting the first predetermined number of bits and allowing 1 to 2 errors in said count.
- Apparatus according to any one of the preceding claims comprising means for counting the second predetermined number of bits and allowing 1 to 4 errors in said count.
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
GB9010711A GB2243977A (en) | 1990-05-12 | 1990-05-12 | Synchronisation apparatus |
GB9010711 | 1990-05-12 |
Publications (2)
Publication Number | Publication Date |
---|---|
EP0456973A2 true EP0456973A2 (en) | 1991-11-21 |
EP0456973A3 EP0456973A3 (en) | 1992-07-08 |
Family
ID=10675917
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
EP19910102981 Withdrawn EP0456973A3 (en) | 1990-05-12 | 1991-02-28 | Synchronisation apparatus |
Country Status (2)
Country | Link |
---|---|
EP (1) | EP0456973A3 (en) |
GB (1) | GB2243977A (en) |
Families Citing this family (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2342015B (en) * | 1998-09-23 | 2003-11-05 | Sony Uk Ltd | Synchronising digital signals |
US7493530B2 (en) * | 2002-06-25 | 2009-02-17 | Avago Technologies General Ip (Singapore) Pte. Ltd. | Method and apparatus for detecting an error in a bit sequence |
Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4435826A (en) * | 1980-09-05 | 1984-03-06 | Hitachi, Ltd. | Frame synchronizer |
JPS59115641A (en) * | 1982-12-22 | 1984-07-04 | Hitachi Ltd | Synchronizing system |
Family Cites Families (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US3586776A (en) * | 1969-04-16 | 1971-06-22 | Motorola Inc | Digital communication synchronization system including synchronization signal termination recognition means |
US4747105A (en) * | 1986-09-03 | 1988-05-24 | Motorola, Inc. | Linear feedback sequence detection with error correction |
-
1990
- 1990-05-12 GB GB9010711A patent/GB2243977A/en not_active Withdrawn
-
1991
- 1991-02-28 EP EP19910102981 patent/EP0456973A3/en not_active Withdrawn
Patent Citations (2)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US4435826A (en) * | 1980-09-05 | 1984-03-06 | Hitachi, Ltd. | Frame synchronizer |
JPS59115641A (en) * | 1982-12-22 | 1984-07-04 | Hitachi Ltd | Synchronizing system |
Non-Patent Citations (2)
Title |
---|
INSTITUTE OF ELECTRICAL AND ELECTRONICS ENGINEERS. INTERNATIONAL CONFERENCE ON COMMUNICATIONS, JUNE 11-13 1973. NEW YORK, PAGES 23-16:23-17 CHARLES C. KILGUS: 'PSEUDO-NOISE CODE ACQUISITION USING MAJORITY LOGIC DECODING' * |
PATENT ABSTRACTS OF JAPAN vol. 8, no. 235 (E-275)(1672) 27 October 1984 & JP-A-59 115 641 ( HITASHI SEISAKUSHO K.K. ) 4 July 1984 * |
Also Published As
Publication number | Publication date |
---|---|
GB9010711D0 (en) | 1990-07-04 |
GB2243977A (en) | 1991-11-13 |
EP0456973A3 (en) | 1992-07-08 |
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